U.S. patent number 5,387,844 [Application Number 08/077,791] was granted by the patent office on 1995-02-07 for flat panel display drive circuit with switched drive current.
This patent grant is currently assigned to Micron Display Technology, Inc.. Invention is credited to Jim J. Browning.
United States Patent |
5,387,844 |
Browning |
February 7, 1995 |
Flat panel display drive circuit with switched drive current
Abstract
A Field Emission Display ("FED") is disclosed having a
brightness to project images. To achieve this benefit, the FED
includes a pixelator which coupled to a display for displaying and
projecting the image. By design, the pixelator conducts a drive
current passing through the display grid corresponding to a degree
of brightness in the resulting panel display. A first resistor
having a first value is coupled between the pixelator and a voltage
node or ground. Moreover, a second resistor having a second value
at most one half of the first value is employed. A switch for
connecting the first resistor in parallel with the second resistor
is closed when a control signal is received when the switch is
enabled, the equivalent resistance between the pixelator and a
voltage node or ground is substantially reduced. In another
embodiment, a tapped resistor replaces the first resistor and the
second resistor. When the control signal is received, a portion of
the drive current is shunted through the switch.
Inventors: |
Browning; Jim J. (Boise,
ID) |
Assignee: |
Micron Display Technology, Inc.
(Boise, ID)
|
Family
ID: |
22140078 |
Appl.
No.: |
08/077,791 |
Filed: |
June 15, 1993 |
Current U.S.
Class: |
315/169.3;
315/169.1; 315/169.4; 315/349 |
Current CPC
Class: |
G09G
3/22 (20130101); G09G 2300/08 (20130101); G09G
2300/0809 (20130101) |
Current International
Class: |
G09G
3/22 (20060101); G09G 003/10 () |
Field of
Search: |
;315/349,169.1,169.3,169.4,311 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Pascal; Robert J.
Assistant Examiner: Philogene; Haissa
Attorney, Agent or Firm: Bachand; William R.
Claims
What is claimed is:
1. A field emission display having a level of brightness, the
display comprising:
a. a pixelator for field emission, the pixelator conducting a drive
current, the drive current corresponding to the level of
brightness;
b. a resistor for providing a resistance between the pixelator and
a node and for establishing the drive current, the resistor having
a first value; and
c. varying means for varying the resistance from the first value,
the varying means enabled by a control signal, thereby varying the
brightness in response to the control signal, wherein the varying
means comprises:
(1) a second resistor having a second value; and
(2) a switch for connecting the resistor in parallel with the
second resistor, the switch being enabled by the control
signal.
2. A field emission display, according to claim 1, wherein the
second value comprises at most one half of the first value.
3. A field emission display having a brightness to project images,
comprising:
a display;
a pixelator for driving said display, said pixelator conducting a
current, said current corresponding to said brightness;
a first resistor for providing a first resistance between said
pixelator and a node, said first resistor having a first value;
a second resistor having a second value, said second value
comprising at most one half of said first value; and
a switch for connecting said first resistor in parallel with said
second resistor, said switch enabled by a control signal.
4. A flat panel display having a pixel, the pixel having a
brightness, pixel display enabled by a drive circuit, the drive
circuit comprising:
a. a first switch for selectively enabling pixel display by
conducting a drive current through the first switch;
b. a first resistor in series with the first switch for providing a
first portion of the drive current, the first resistor comprising a
tap; and
c. a second switch, coupled to the tap, for shunting a portion of
the first resistor in response to a control signal so that the
brightness is responsive to the control signal.
5. The flat panel display of claim 4 wherein the drive circuit
further comprises a field emitter tip.
6. The flat panel display of claim 5 wherein the second switch
comprises a field effect transistor having a gate coupled to the
control signal.
7. The flat panel display of claim 5 wherein the first portion of
the drive current enables directly viewing an image on the display
and the second portion of the drive current enables projecting the
image onto a background for indirectly viewing the image.
8. A flat panel display having a pixel, the pixel having a
brightness, pixel display enabled by a drive circuit, the drive
circuit comprising:
a. a first switch for selectively enabling pixel display by
conducting a drive current through the first switch;
b. a first resistor in series with the first switch for providing a
first portion of the drive current, the first resistor comprising a
tap; and
c. a current source coupled to the tap, the current source for
providing, when selectively enabled by a control signal, a second
portion of the drive current so that the brightness is responsive
to the control signal.
9. The flat panel display of claim 12 wherein the drive circuit
further comprises a field emitter tip.
10. The flat panel display of claim 9 wherein the current source
comprises a transistor for conducting the second portion of the
drive current in response to the control signal.
11. The flat panel display of claim 10 wherein the second current
source further comprises a second resistor coupled in series with
the transistor so that the second portion of the drive current
passes through the second resistor.
12. A flat panel display having a pixel, the pixel having a
brightness, pixel display enabled by a drive circuit, the drive
circuit comprising:
a. a first switch for selectively enabling pixel display by
conducting a drive current through the first switch;
b. a first current source in series with the first switch for
providing a first portion of the drive current; and
c. a second current source coupled in parallel with the first
current source, the second current source for providing, when
selectively enabled by a control signal, a second portion of the
drive current so that the brightness is responsive to the control
signal.
13. The flat panel display of claim 12 wherein the drive circuit
further comprises a field emitter tip.
14. The flat panel display of claim 13 wherein the first current
source comprises a first resistor.
15. The flat panel display of claim 14 wherein the second current
source further comprises a second resistor coupled in parallel with
the first resistor so that the second portion of the drive current
passes through the second resistor.
16. The flat panel display of claim 14 wherein the first current
source comprises a tapped resistor coupled to the second source for
conducting the second portion of the drive current through the
tap.
17. The flat panel display of claim 13 wherein the second current
source comprises a second switch for conducting the second portion
of the drive current in response to the control signal.
18. The flat panel display of claim 10 wherein the second switch
comprises a field effect transistor.
19. The flat panel display of claim 13 wherein the first portion of
the drive current enables directly viewing an image on the display
and the second portion of the drive current enables projecting the
image onto a background for indirectly viewing the image.
20. A field emission display comprising:
a. a first pixel having a first brightness, the first pixel enabled
for display by a first switch, the first switch for selectively
passing a first drive current;
b. a second pixel having a second brightness the second pixel
enabled for display by a second switch, the second switch for
selectively passing a second drive current;
c. a first current source coupled to the first switch and coupled
to the second switch, the first current source for providing a
first portion of both the first drive current and the second drive
current; and
d. a second current source coupled in parallel with the first
current source, the second current source for providing, when
selectively enabled by a control signal, a second portion of both
the first drive current and the second drive current so that the
first brightness and the second brightness are responsive to the
control signal.
21. The field emission display of claim 20 wherein:
a. the first switch comprises a first transistor responsive to a
row signal and a second transistor responsive to a first column
signal; and
b. the second switch comprises a third transistor responsive to the
row signal and a fourth transistor responsive to a second column
signal.
22. The field emission display of claim 21 wherein the first
portion of the drive current enables directly viewing an image on
the display and the second portion of the drive current enables
projecting the image onto a background for indirectly viewing the
image.
Description
FIELD OF THE INVENTION
The present invention relates to flat panel displays and, more
particularly, to an apparatus for switching the brightness of a
flat panel display.
BACKGROUND OF THE INVENTION
Until recently, the cathode ray tube ("CRT") has been the primary
device for displaying information. While having sufficient display
characteristics with respect to color, brightness, contrast, and
resolution, CRTs are relatively bulky and power hungry. In view of
the advent of portable laptop computers, the demand has intensified
for a display technology which is lightweight, compact, and power
efficient.
One available technology is flat panel displays, and more
particularly, Liquid Crystal Display ("LCD") devices. LCDs are
currently used for laptop computers. However, those LCD devices
provide poor contrast in comparison to CRT technology. Further,
LCDs offer only a limited angular display range. Moreover, color
LCD devices consume power at rates incompatible with extended
battery operation. In addition, a color LCD type screen tends to be
far more costly than an equivalent CRT.
In light of these shortcomings, there have been several
developments recently in thin film, Field Emission Display (FED)
technology. In U.S. Pat. No. 5,210,472, commonly assigned with the
present invention, an FED design is disclosed which utilizes a
matrix-addressable array of pointed, thin-film, cold emission
cathodes in combination with a phosphor luminescent screen. There,
the FED incorporates a column signal to activate a single
conductive strip within the cathode grid, while a row signal
activates a conductive strip within the emitter base electrode. At
the intersection of both an activated column and an activated row,
a grid-to-emitter voltage differential exists sufficient to induce
a field emission, thereby causing illumination of the associated
phosphor of a pixel on the phosphorescent screen. Extensive
research has recently made the manufacture of an inexpensive, low
power, high resolution, high contrast, full color FED a more
feasible alternative to LCDs.
In light of its inexpensive, low power, full color, high
resolution, high contrast capabilities, several new applications of
FED technology are currently being explored. One area of interest
is utilizing FEDs in the projection of images. For example, in the
area of video camera technology, where a viewfinder displays the
captured image within a channel designed for close viewing, there
has been a growing interest in projecting the captured image onto a
background. Presently, FEDs display images by illuminating a pixel
on the phosphorescent screen. Nonetheless, the energy generated by
the FED in the process of illumination is insufficient to project
an image from the display onto a background.
SUMMARY OF THE INVENTION
The primary advantage of the present invention is to eliminate the
aforementioned drawbacks of the prior art.
A further advantage of the present invention is to provide an
apparatus for switching the brightness of a flat panel display.
Another advantage of the present invention is to provide an FED
that can display and project images.
In order to achieve these hereinabove advantages, as well as others
which will become apparent hereafter, a field emission display
("FED") of the present invention has a variable brightness to
project images. To achieve this benefit, the FED includes a
pixelator coupled to a display for displaying and projecting the
image. By design, the pixelator conducts a drive current passing
through the display grid corresponding to a degree of brightness in
the resulting panel display. In a first embodiment of the present
invention, a voltage controlled resistor is coupled between the
pixelator and a voltage node or ground. In a second embodiment, a
first resistor having a first value, is coupled between the
pixelator and a voltage node or ground. Moreover, a second resistor
having a second value at most one half of the first value is
employed. A switch for connecting the first resistor in parallel
with the second resistor is closed when a control signal is
received. When the switch is enabled, the equivalent resistance
between the pixelator and a voltage node or ground is substantially
reduced. In a further embodiment of the invention, the first
resistor comprises a resistive layer, while the second resistor
comprises a tap for tapping the resistive layer between the first
and second terminations of the resistive layer, thereby creating
the second resistor smaller than the first resistor.
Other aspects and advantages will become apparent to those skilled
in the art from the following detailed description read in
conjunction with the appended claims and the drawings attached
hereto.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be better understood from reading the
following description of non-limitative embodiments, with reference
to the attached drawings, wherein below:
FIG. 1 is a schematic diagram of a field emission display device
employing a first embodiment of the present invention;
FIG. 2 (a) is a schematic diagram of a field emission display
device employing a second embodiment of the present invention, FIG.
2 (b) is a diagrammatic view of a physical realization of the
second embodiment, while FIG. 2 (c) is a alternate realization of
the second embodiment;
FIG. 3 is a schematic diagram of a field emission display device
employing a third embodiment of the present invention; and
FIG. 4 is a schematic diagram of a field emission display device
employing a fourth embodiment of the present invention.
It should be emphasized that the drawings of the instant
application are not to scale but are merely schematic
representations and are not intended to portray the specific
parameters or the structural details of the invention, which can be
determined by one of skill in the art by examination of the
information herein.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 1, a Field Emission Display ("FED") device 10 is
illustrated employing a first embodiment of the present invention.
Device 10 comprises a series of field emitter tips 20 and 20' and a
display grid 15. Relying on the principles of FED technology, as
described in U.S. Pat. No. 5,210,472 and incorporated herein by
reference, electrons are emitted via tips 20 and 20' and through
grid 15 in order to illuminate a phosphorus background (not shown)
and display an image.
Incorporated with the field emitter tips 20 and 20' and a display
grid 15 are pixelators 25 and 25'. Pixelators 25 and 25' each have
a first termination coupled to a tip 20 or 20', and are enabled by
means of a row control and a column control signal. Once enabled,
pixelators 25 and 25' drive field emitter tips 20 and 20' by means
of a drive current, acting as a constant current source for device
10. Further, a dependent relationship exists between the drive
current associated with each pixelator and the brightness
associated with that emitter tip.
In order for proper operation, each pixelator, 25 and 25',
comprises a resistance coupled between its second termination and
ground through which its drive current is conducted. This
resistance can be either a discrete resistor or a layer of material
having a predetermined resistivity. As each pixelator, 25 and 25',
acts as a constant current source, given a resistance having a
predetermined value, the drive current supplied to its coupled
emitter tip will be a known, quantifiable value.
Under the arrangement described hereinabove, the drive current is
limited by the value of the gate resistance interposed between the
gate terminals of the pixelator and ground. However, in the event
device 10 was required to project an image, a greater brightness
and luminescence would be required. Given the relationship between
drive current and brightness, a means for varying the drive current
is needed in order to project an image on a background.
In order to address this particular need, several realizations are
available. In FIG. 1, a voltage controlled resistance 30 and 30' is
utilized between the second termination of each pixelator, 25 and
25', and a voltage node or ground. Enabled by a control signal 35,
this design provides a controlled means for varying the drive
current resistance. Thus, voltage controlled resistance 30 and 30'
can provide several degrees of brightness, the greatest being when
device 10 is chosen for projection purposes. In this scenario, the
control signal enables an extremely low resistance value from
voltage controlled resistance 30 and 30', thereby providing the
maximum available drive current through each pixelator, 25 and 25',
while maintaining the integrity and functionality of device 10.
Referring to FIGS. 2 (a), (b), and (c), a second embodiment of the
present invention is provided. For realizing the means for varying
the drive current in order to project an image on a background by
way of a voltage controlled resistance, a tapped resistance is
employed.
With respect to FIG. 2 (a), a preferred filed emission display
device 11 design is shown of a drive current resistance 40 with a
layer of material having a predetermined resistivity interposed
between the second termination of each pixelator, 25 and 25', and a
voltage node or ground. To lower the effective resistance between
each pixelator and a voltage node or ground, each resistance 40 and
40' is tapped at some point by one conductor of a switch 50 and
50'. Once enabled by a control signal 35, a second conductor of
each switch, 50 and 50', conductively taps each pixelator's
associated resistance, 40 and 40'. However, it should be obvious to
one of ordinary skill in the art that the second conductor of each
switch, 50 and 50', could conductively tap the base voltage node or
ground. Each Switch 50 and 50', preferably comprising a field
effect transistor, acts as a shunt by tapping resistance 40 to
reduce the effective resistance viewed by each pixelator.
Referring in FIG. 2 (b), resistance 40 is shown in greater detail.
Resistance 40 comprises a layer 55 having a first and second
termination, 60 and 65, whereby first termination 60 is coupled
with pixelator 25 and second termination 65 is coupled with a
voltage node or ground. Between the first and second terminations,
60 and 65, a conductive tap 70 is used. Tap 70 is employed for
tapping the resistive layer 55. By this arrangement, the effective
resistance viewed by pixelator 25 is reduced according to the
position of tap 70 along layer 55. This positioning is dependent on
design considerations associated with the resistance, as well as
the operating current necessary to drive switches 50 and 50'. As
such, the resistance created between the tapping point and second
termination is preferably greater than the resistance between the
tapping point and the first termination. As described above,
conductive tap 70 is enabled by switch 50 through control signal
35.
In FIG. 2 (c), drive current resistance 40 and 40' is shown each
comprising a layer of material having a predetermined resistivity,
Resistance 40 and 40' is interposed between the second termination
of each pixelator, 25 and 25', and a voltage node or ground, as
described above. To lower the effective resistance between each
pixelator and a voltage node or ground, each resistances 40 and
40', is tapped at some point by one conductor of a switch 51 and
51'. Once enabled by a control signal 35, a second conductor of
each switch, 51 and 51', is conductively coupled with a resistor,
52 and 52'. Each resistor, 52 and 52', is coupled to a base voltage
node or ground commonly shared with the second termination of
resistances, 40 and 40'. However, it should be obvious to one of
ordinary skill in the art that the second conductor of each switch,
51 and 51', could be conductively coupled to the node where each
pixelator is coupled with its associated resistance, 40 and
40'.
It should be noted that design considerations factor into the
actual values associated with resistors, 52 and 52'. Each switch 51
and 51', preferably comprising a field effect transistor, acts as
shunt by tapping resistance 40 to reduce the effective resistance
viewed by each pixelator. Should switches 51 and 51' be realized by
field effect transistors, the values considered for resistors, 52
and 52', must maintain the stability of the overall device 12, the
pixelators 25 and 25', as well as the region for which the
transistor operates as a switch.
Referring to FIG. 3, a third embodiment of the present invention is
shown. In place of tap 70, this embodiment employs a discrete drive
current resistor 75 between each pixelator, 25 and 25', and ground.
Further, a second resistor 80 is provided in parallel with drive
current resistor 75. However, second resistor 80 conducts current
only when switch 50, preferably comprising a field effect
transistor, is enabled. Switch 50 is enabled by means of control
signal 35. It should be obvious to one of ordinary skill in the art
that this same structure applies to each pixelator employed in
device 13.
Referring to FIG. 4, a fourth embodiment of the present invention
is illustrated. For reducing the overall size of device 14
employing the present invention, one drive current resistor 90 is
employed for all pixelators used in device 14. Further, a second
resistor 85 is provided in parallel with drive current resistor 90
by means of switch 50, which preferably comprises a field effect
transistor. Switch 50 allows current to pass through second
resistor 85 upon receiving control signal 35. As before, the
effective or equivalent drive current resistance viewed by the
pixelators is substantially reduced. It should be noted that this
particular embodiment is pertinent where discrete component
resistors are used.
By employing any of the embodiments described herein, the drive
current resistance is substantially reduced when control signal 35
is enabled. To achieve this end, the second resistance must be at
most one half of the value of the drive current resistance to
substantially reduce the effective drive current resistance. By
this approach, the effective drive current is substantially
increased thereby enabling device 10-14 to project images onto a
background, such as a wall.
The primary purpose of substantially reducing the drive current
resistance is directed to uses where device 10-14 is switched into
a projection mode of operation. Other modes for operating device
10-14, however, are conceivable. For example, when device 10-14 is
being viewed in an environment not conducive to viewing, a greater
brightness may be required than that needed in its normal expected
environment.
While the particular invention has been described with reference to
illustrative embodiments, this description is not meant to be
construed in a limiting sense. It is understood that although the
present invention has been described in a preferred embodiment,
various modifications of the illustrative embodiments, as well as
additional embodiments of the invention, will be apparent to
persons skilled in the art upon reference to this description
without departing from the spirit of the invention, as recited in
the claims appended hereto. For example, the present invention
pertains to flat panel display, and more particularly, FEDs.
Nonetheless, the inventive features described herein can also be
incorporated in LCD technology. It is therefore contemplated that
the appended claims will cover any such modifications or
embodiments as fall within the true scope of the invention.
All of the U.S. Patents cited herein are hereby incorporated by
reference as if set forth in their entirety.
* * * * *